The present invention relates to a drive for cooling fans in motor vehicles. German Patent Application DE-OS 14 25 374 describes a cooling fan including a main cooling circuit including a main cooler and at least one secondary cooling circuit each including a secondary cooler, including a fluid friction clutch including a driving clutch unit being drivingly connected to a drive shaft, and a driven clutch unit in which a reservoir for a viscous fluid is located, the reservoir being limited by a disk-like separating wall and being connectable to a working chamber by an opening in the separating wall, the working chamber extending into a region between the clutch units in which torque is transmitted form the driving clutch unit to the driven clutch unit and the filling of which with viscous fluid is controlled by a control element freeing the opening in the separating wall depending on the temperature of the cooling air streaming through the main cooler being determined by a temperature sensor.
In the cooling fan described in DE-OS 14 25 374, the control element is designed as a pivotable valve disc closely fitting the separating wall at the side facing the reservoir chamber and being connected to a temperature sensor in the form of a spiral-like bimetallic strip. The temperature sensor is substantially exposed to the stream of outgoing air of the cooler being arranged in the main cooling circuit and effecting the cooling of the motor and it pivots the valve disc depending on the temperature of the stream of outgoing air in a way such that the valve is opened at or above a certain temperature, such that viscous fluid may enter the working chamber, the fluid effecting a transmission of torque from the driving to the driven clutch unit. With a liquid friction clutch controlled this way, the power of cooling fans is adaptable to the prevailing conditions of operation. Additionally, unnecessary consumption of energy by the cooling fan reducing the usable power of the motor is prevented.
This is only true in case of the stream of outgoing air being substantially homogeneous across its cross section since the fluid friction clutch only reacts to the temperature of the stream of outgoing air contacting the temperature sensor. In case the stream of outgoing air is strongly inhomogeneous across its cross section since it consists of the streams of outgoing air of coolers being strongly differently loaded by heat, it is possible that the fluid friction clutch does not react to drive the cooling fan as desired. In this way, it is possible that the stream of outgoing air of the main cooler to which the temperature sensor is subjected has a temperature which is too low during the operation of the vehicle to effect an actuation of the cooling fan, while the heated stream of outgoing air of a secondary cooler for hydraulic oil hydraulically driving a strongly loaded device does not reach the temperature sensor.